1. Field of the Invention
The present invention relates generally to an integrated Global Positioning System/Inertial Navigation System (GPS/INS) apparatus provided with a plurality of GPS antennas fixedly mounted on a moving body (i.e., a mobile vehicle like a ship) and an inertial sensing system and, more particularly, the invention is concerned with a method of examining and validating integer ambiguity candidates by using attitude information in an integrated GPS/INS apparatus.
2. Description of the Related Art
An attitude determination apparatus for determining the attitude of a moving body by using a plurality of GPS antennas and an inertial sensing system is an example of a conventional carrier phase-based relative positioning apparatus. Typically, an attitude determination apparatus including a plurality of GPS antennas and inertial sensors rigidly fixed to a moving body measures carrier phase differences from signals received from multiple GPS satellites by antenna pairs, calculates position vectors (or baseline vectors) of the individual antennas except for one used as a reference antenna relative to the reference antenna from carrier phase difference observables, and determines the attitude of the moving body from the baseline vectors.
In this kind of relative positioning based on carrier phase differences, it is necessary to determine integer ambiguities before calculating baseline vectors. Fast determination of correct integer ambiguities is a main technical issue in carrier phase-based relative positioning. Techniques for fast integer ambiguity resolution are described in the following patent and non-patent publications, for example:    (1) U.S. Pat. No. 6,611,228;    (2) R. R. Hatch, “Ambiguity Resolution in the Fast Lane,” Proceedings of the Second International Technical Meeting of the Institute of Navigation, Colorado Springs, Colo., 1989;    (3) P. J. G. Teunissen, “A New Method for Fast Carrier Phase Ambiguity Estimation,” Proceedings IEEE Position, Location and Navigation Symposium PLANS '94, Las Vegas, Nev., April 11-15, pp. 562-573.
Determination of integer ambiguities is generally a two-step procedure involving determination of the sets of integer ambiguity candidates and validation of the sets of integer ambiguity candidates. Integer ambiguity resolution for obtaining integer ambiguity candidate sets is a process of calculating float solutions of integer ambiguities and then determining p number of integer ambiguity candidate sets (N1, N2, . . . , Nk)p. Validation of the integer ambiguity candidate sets is a process of examining the integer ambiguity candidate sets and selecting one set of correct integer ambiguities.
Generally speaking, various kinds of a priori information are used for validation of the integer ambiguity candidate sets. Typically, validation of the integer ambiguity candidate sets is accomplished by using such a priori information as known information on baseline lengths or information on antenna geometry including the inner product or cross product of each pair of baseline vectors as described in U.S. Pat. No. 6,611,228 on “Carrier Phase-Based Relative Positioning Apparatus.” According to the Publication, this ambiguity validation approach yields a probability of correct integer estimation, or success rate, of approximately 90% to 95%.
To examine and validate the integer ambiguity candidate sets by using the a priori information on all the aforementioned antenna geometry, however, it is necessary to obtain a plurality of baseline vectors with at least three antennas rigidly mounted on a moving body. If one attempts to examine and validate the integer ambiguity candidate sets with a single baseline obtained from two antennas only, the a priori information on the antenna geometry available for ambiguity validation is for that baseline length only and, under this circumstance, the ambiguity validation approach of the aforementioned Publication can provide a correct integer ambiguity candidate set at a much decreased success rate. Especially when only a small number of GPS satellites are available or when measurements are affected by multipath disturbance, there arises a problem that the success rate of identifying a correct integer ambiguity candidate set decreases to an unacceptably low level for practical use.
On the other hand, in a carrier phase-based relative positioning apparatus developed with special emphasis on compact design, it is particularly important to achieve a nearly 100% success rate of obtaining a correct integer ambiguity set and, thus, there is a growing demand for an improved ambiguity validation technique.
U.S. Pat. No. 6,611,228 describes an ambiguity validation technique for examining and validating integer ambiguity candidates by using information on the attitude of a moving body, the technique involving a process of examining the integer ambiguity candidates by using attitude information delivered from an INS apparatus. This ambiguity validation technique has a problem that the attitude information can not be obtained from the INS apparatus until integer ambiguities are first determined, so that the attitude information can not be used for examining and validating the integer ambiguity candidates in an initial stage.